1. Field of the Invention
This invention relates to a process of briquetting sponge iron-containing material which has been produced by direct reduction and is in a cooled state and contains a binder added before the material is briquetted.
2. Discussion of the Prior Art
Sponge iron-containing materials become available as a result of the direct reduction or waelz processing of materials which contain iron oxides by means of solid carbonaeous reducing agents or reducing gases in a rotary kiln, shaft furnace or retort furnace. Such materials may particularly consist of iron ores or of residual materials obtained in metallurgical operations. The material which contains sponge iron may be treated to remove other constitutents of the charge and is then processed to produce crude iron or steel. For that purpose the material may be charged to low shaft electric furnaces, blast furnaces or electric arc furnaces. Whereas such charges must have a particle size above are lower limit, the sponge iron-containing material which becomes available has in part or entirely a particle size below said lower limit. Additionally, the fines, which generally consist of particles below about 6 mm, are highly susceptible to atmospheric influences in storage and transit. It would be essential to avoid an exposure to water but this cannot always be ensured. Moreover, the handling of material which contains sponge iron, inclusive of a substantial proportion of fines always involves a disturbing raising of dust.
These disadvantages can be eliminated by a decrease of the surface area of the material. This can be accomplished by briquetting, which has been known for a long time. The resulting formation of larger lumps is also a requirement for the use of such materials in a blast furnace. Briquetting may be effected at high or low temperatures. It is desired to effect it at one heat, although this is not possible in many cases. In cold briquetting, the wear of tools depends highly on the pressure applied and this can be decreased by an addition of certain binders and by providing the required strength of the compact article by a subsequent age hardening. In the selection of the binders, their availability, price and proportioning properties, the age hardening time required and the behavior of the compact during the succeeding heat treatment must be taken into account. Known binders which are acceptable also from the cost aspect include lime, molasses, spent sulfite cooling liquor, bitumen and starch, although all of them require an age hardening for at least 24 hours. Some of them cause the compacts to disintegrate during the succeeding heat treatment.
German Pat. No. 1,071,733 discloses a process of cold-briquetting sponge iron-containing material without an addition of a binder. The briquette products are treated with CO.sub.2 in the persence of moisture. The moist briquettes which contain CO.sub.2 are then aged in the presence of an oxygen-containing gas. That process is expensive and takes considerable time.
It is known from German Pat. No. 1,270,054 to cool all matter discharged from the furnace in such a manner that a formation of martensite is substantially avoided, and then to briquette the material without a binder. In that process, a special cooling of the sponge iron-containing material is required.
From "Neues aus der Technik", Vogel-Verlag, Wurzburg, 1977, No. 2, page 1, it is known that the matter discharged from a rotary kiln after waelz processing can be briquetted. Spent sulfite sooling liquor is added to the matter which has been discharged so that the solids of the spent sulfite amount to 2 to 5% of the matter which has been discharged. The mixture is aged and then briquetted. The briquettes are age-hardened at room temperature for at least 2 hours and preferable for 12 to 72 hours. That process requires ageing and age hardening steps.
From Austrian Pat. No. 221,588 and German Pat. Nos. 1,140,592 and 1,143,837 it is known that fine-grained ore, blast furnace flue dust or similar pulverulent materials which contain not more that 15% metallic iron in the form of a very fine powder can be briquetted after an addition of sulfuric acid and/or aqueous iron sulfate solutions. Thereafter, the briquettes are watered and subsequently age-hardened in the presence of air. An increase of the metallic iron content above 13% does not result in an appreciable improvement in strength and involves an inadequate initial strength. For this reason, pre-reduced ores which contain more metallic iron are mixed with other materials so that the mixture contains between 2% and about 10% of metallic iron.
It is an object of the invention to provide a process for the briquetting of sponge iron produced by a direct reduction so that the briquettes need not be after-treated before being charged to metallurgical furnaces.